Why does Google Chrome not run as root user by default in Kali Linux? Is it a security issue?
It's basically because of the principle of least privilege (see for example: https://en.wikipedia.org/wiki/Principle_of_least_privilege). There is no reason to give Chrome all the power and capabilities that root has. More privileges mean more trouble in case something goes wrong.
As a side note I have to say though, that I am skeptical about the advantages of low privileges in the specific case a typical Linux desktop gets compromised, because the typical desktop user regularly uses
sudo to basically get all the privileges. The user just trusts sudo, but sudo is just meant to be used for convenience, not for real security, because it doesn't provide enough separation of privileges. But this is a side note, regarding a specific issue. In general the principle of least privilege always applies.